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IRON                                           177

treated in a beaker in the cold with 20 c.c. of 10% barium chloride solution.
The volume is made up to about 100 c.c. with water, the liquid stirred
and, after standing for 12-24 hours, filtered by decantation and the pre-
cipitate finally transferred to the filter and washed alternately with hot
dilute hydrochloric acid (10%) and cold water until the washings no longer
give the iron reaction with thiocyanate. The barium sulphate thus
obtained, which should be perfectly white, is dried, ignited and weighed :
BaS04 x 0-1374 = S (in 5 grams of sample).

This method is moderately delicate. In presence of large quantities of iron
the precipitation may sometimes be incomplete or the barium sulphate may
contain iron as an impurity. This difficulty is obviated by Meinecke x and
by Carnot and Goutal2 by the following method (see b), the iron being removed
by means of copper-potassium chloride, and the oxidation of the sulphur being
made on the insoluble residue, which contains the sulphur as sulphides of copper
and iron and only very little of the latter.

finely powdered sample are heated, with frequent shaking, with about 50
grams of copper-potassium chloride and 250 c.c. of water (see Determination
of carbon by means of copper chloride) for about 15 minutes on a water-
bath, 10 c.c. of hydrochloric acid being then added and the liquid again
heated to dissolve the separated metallic copper. The insoluble residue
is then collected on a small paper or asbestos filter and washed with hot
water. The filter and precipitate are then evaporated on a water-bath
to dryness in a small dish with a little potassium chlorate, 5 c.c. of nitric
acid (D 1-4) and 10 c.c. of hydrochloric acid (D 1-19). The residue is then
taken up with a little hydrochloric acid, again heated almost to dryness
on a water-bath, diluted, and filtered and washed with hot water. The
filtrate is neutralised with ammonia, acidified slightly with hydrochloric
acid and precipitated in the hot with barium chloride.

This method is sufficiently exact and rapid.

2. Volumetric Determination of the Sulphur (Rollet & Campredon's
method).—When wrought-iron, cast-iron or steel is acted on by hydrochloric
or sulphuric acid, the sulphur present is liberated mostly as hydrogen sulphide
and in small part as sulphur compounds [especially (CH3)2S], which may
be reduced to hydrogen sulphide by heating in presence of hydrogen in a
porcelain tube. The hydrogen sulphide is fixed by zinc acetate and the
sulphide formed determined iodometrically.

Reagents: (i) 25 grams of pure, crystallised zinc acetate and i c.c. of
acetic acid dissolved to i litre.

(2) 7-928 grams of resublimed iodine are dissolved with the help of
25 grams of potassium iodide and the volume made up to i litre: 3 i c.c.
corresponds with o-ooi gram of sulphur.4

1 Zeitschr. angew. Chem., 1888, p. 376.                         2 Compt, vend,, 1897.

3  The titre of the iodine solution may be controlled by comparing the thiosulphate
solution with iodine by the method used for determining the iodine number of fatty
substances (see chapter on Fatty Materials) and then titrating the thiosulphate and
iodine solutions.

4  The reaction between zinc sulphide and iodine takes place thus :   Zn S + 2! =
ZnI2 + S, so that 32 parts of sulphur require 257-7 Parts of iodine or i gram of sulphur,

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